When I first started testing out the ECG feature on the Apple Watch Series 4 — which launched in December with the update to Watch OS 5.1.2 — the last thing I expected was to find something abnormal about my heart rhythm. But that’s exactly what happened when I was cross-referencing the Watch’s readings with medical-grade EKG equipment at the doctor’s office. “We see on your Apple Watch the same early heartbeat that we see on the EKG,” said Dr. Gregory Marcus, professor of medicine and a cardiac electrophysiologist at UCSF Medical Center, as I sat on the hospital bed with cables attached to my body and an Apple Watch Series 4 on my wrist…

An EKG uses electrodes to measure the electrical activity of the heart. A hospital-grade EKG generally consists of 10 electrodes placed on different parts of your body. The Apple Watch Series 4 has two: one electrode on the back crystal and one electrode on the digital crown.

As he was looking at his monitor, I opened the new ECG app (Apple uses the abbreviation ECG, whereas doctors generally say EKG) on the Apple Watch Series 4 to take my first EKG. I put my finger on the digital crown and waited while the screen counted down 30 seconds… The EKG on the Apple Watch directly coincided with the results of the printed-out hospital EKG.